bugc: the optimizer's fixpoint loop compares JSON that leaves out every block #386

optimizeIr (packages/bugc/src/optimizer/simple-optimizer.ts) repeats the pipeline at -O2 and up "until fixpoint", and it compares rounds by JSON.stringify(current). But a function's blocks are a Map, and so is module.functions, and JSON.stringify writes a Map as {}. The comparison does not see the code at all.

Counting the rounds (a counter in the loop), every program I tried at -O3 runs exactly two: the first round changes something outside the Maps, the second does not, and the loop stops. Whether the second round changed the code, and whether a third would, makes no difference.

So the passes run twice, not to a fixpoint. An optimization that needs a third round (one pass's result enabling another's, two rounds down) does not happen. And tests that count on the fixpoint may test less than they say.

A fix is to compare something that includes the blocks: for example, a serialization that writes Maps as entries, or a "changed" flag from each pass.